Integrated Circuits (ICs) are commonly packaged in surface-mount package types such as a Ball Grid Array (BGA) package. A BGA is a surface-mount package that utilizes an array of metal spheres or balls as the means of providing external electrical interconnection. The balls are composed of solder, and are attached to a Printed Circuit Board (PCB) substrate at the bottom side of the package. The IC die is mounted on the top side of the package and is connected to substrate bond pads either by wire-bonding or flip-chip connection. The PCB substrate of the package often has internal conductive copper traces that route and connect the substrate bond pads to the solder balls.
FIG. 1 (Prior Art) illustrates a simplified cross-section view of a BGA package 10. BGA package 10 includes an integrated circuit (IC) chip or die 11, a substrate 12, an external solder ball 13, and encapsulant 14. Substrate 12 includes a copper trace 15 and a bond pad or finger 16. As illustrated in FIG. 1, bond wire 17 couples an active surface of die 11 to bond pad 16, and copper trace 15 couples bond pad 16 to solder ball 13 through via 18.
Substrate bond pad 16 is normally electroplated with a layer of gold because of the excellent conductive and mechanical properties of gold. In order to facilitate the electroplating process, manufactures use tie bars to provide electrical paths to the bond pads. FIG. 2 is a top view of a portion of package 10 of FIG. 1. As illustrated in FIG. 2, copper trace 15 extends toward a side edge 20 of substrate 12 and connects to a tie bar 19. Bond pad 16 is electrically connected with other bond pads by tie bar 19 so that all the bond pads can be electroplated at the same time with a layer of gold.
FIG. 3 illustrates a perspective view of the BGA package 10 of FIG. 1. As illustrated in FIG. 3, copper trace 15 extends to side edge 20 and is visible from the side of BGA package 10. Therefore, this packaging method does not prevent a possible attacker from gaining access to IC buses on the IC chip through copper traces that extend to side edges of the substrate and are visible from the sides of the package. The attacker can probe these exposed copper traces and can learn the data on these buses and may, for example, cause a central processing unit (CPU) of the IC chip to execute malicious code. Some IC manufacturers have developed sophisticated algorithms and data encryption techniques that secure these transactions. However, more and more people are learning the algorithms and encryption methods and are able to break through the security. Therefore, it is beneficial to make it more difficult for possible attackers to access the IC buses. It is especially desirable to secure access to the IC buses designated for security sensitive transactions.